Cast billets each having a round or angular cross section, going through steps of tubemaking and rolling, are used as materials of seamless pipes and shape steels having different sizes in cross section. Since the seamless pipes and shape steels have various kinds of product sizes and different rolling steps, the cast billets to be their base materials also have a variety of cross-sectional shapes. Therefore, a casting in which the number of casting mold is determined depending on production capacity is carried out.
Here, among cast slabs produced by means of a continuous casting or among rolling steel ingots after an ingot casting, a cast slab or ingot having a regular-square cross section or round cross section is defined as a billet, and a cast slab or ingot having a rectangle cross section is defined as a bloom. Also, in the billet, a billet having a regular-square cross section is defined as a square billet, and a billet having a round cross section is defined as a round billet.
A continuous casting will be described with reference to FIG. 1 that is a longitudinal cross-sectional view of a configuration example of a continuous casting system 100 for billet to which the present invention can be applied, wherein the continuous casting system 100 is seen from a lateral side. In FIG. 1, 1 is a tundish, 2 is a molten steel, 3 is a submerged nozzle, 4 is a casting mold, 5 is an electromagnetic stirrer, 6 is a casting roll positioned right below the casting mold, 7 is a zone of roller aprons including a secondary cooling spray zone, 8 is a solidifying shell, 9 is pinch rolls, and 10 is a cast slab.
In the continuous casting, the molten steel 2 poured from a ladle to the tundish 1 is teemed to the casting mold 4 via the submerged nozzle 3. While the molten steel 2 teemed to the casting mold 4 is drawn along a group of casting rolls 6 by the rotational drive of the pinch rolls 9, surface of the solidifying shell 8 is cooled by the second cooling spray zone to proceed solidification, whereby the cast slab 10 is made.
In the continuous casting, it is extremely important to control flow of molten steel in a casting mold in view of operation and quality of cast slab, for instance in view of melt stabilization of mold powder by supplying heat to meniscus and inclusion removal at a surface of cast slab. As a method for controlling flow of molten steel in a casting mold, an electromagnetic stirring applying electromagnetic force to the molten steel in the casting mold and stirring the molten steel is widely known. In a case where the electromagnetic stirring is operated with a plurality of casting molds, it is necessary to apply the electromagnetic force to each of the plurality of casting molds such that the casting molds have a uniform flow.
As methods for applying the electromagnetic force for electromagnetic stirring, a rotational shifting magnetic field type and a linear shifting magnetic field type are exemplified.
The rotational shifting magnetic field type is applied to continuous castings of billet, bloom and the like, and the rotational shifting magnetic field type is a method to obtain a uniform flow by applying a rotating magnetic field to inside of casting mold by means of a plurality of magnetic poles provided along whole circumference of the casting mold (for example, Patent Document 1).
However, in a case where the rotational shifting magnetic field type is applied to a plurality of casting molds, since an electromagnetic stirrer is needed for each of the casting molds, the number of installation of the electromagnetic stirrer is increased and the plurality of casting molds become unable to share a strand due to increase in size of the casting molds, which causes increase in equipment cost.
On the other hand, as the linear shifting magnetic field type, the applicant of the present invention has proposed, in Patent Document 2, an electromagnetic coil in which two of tooth 12 are provided to a core 11 of an iron core of a coil in a projecting manner to a side of a casting mold 4, an inner winding is applied to each of the two of tooth 12, and in addition, an outer winding is applied to the outside of the two of tooth 12 to unify the two of tooth 12. The electromagnetic coil proposed in Patent Document 2 will be described with reference to FIG. 2A. This electromagnetic coil shifts a magnetic field in a linear manner, by applying three-phase alternating currents A, B and C each having a phase difference of 120° to each other to an inner winding 13 and an outer winding 14 as shown in FIG. 2A. Hereinafter, this electromagnetic coil is referred to as a pie-shaped electromagnetic coil.
An electromagnetic stirrer including this pie-shaped electromagnetic coil has a large magnetic flux since the magnetic field in a phase where the outer winding is applied goes in the same direction, and in a case where an electromagnetic force is applied to a casting mold having a large cross section, it is possible to obtain a favorable electromagnetic force along whole circumference of the casting mold (see FIG. 6A).
However, in a case where a plurality of casting molds each having a small cross section are installed between the pie-shaped electromagnetic coils, since the space L between the pie-shaped electromagnetic coils becomes narrow, the magnetic flux component going through the casting mold 4 becomes too strong, whereby shifting magnetic field becomes difficult to be made, which results in a creation of a discontinuous region in the electromagnetic force (see the distortion of the electromagnetic force at the non-uniform flowing part in FIG. 6B).